Recently there has been an increase in the use of devices such as gas turbines in thermal power plants, various reactors in chemical industries, atomic energy facilities, etc., which are to be operated at high temperatures. Accordingly, there has been an increase in the importance of obtaining heat-resistant metals to be used as structural materials. Metal materials used to construct such devices must function at temperatures as high as 600.degree.-1000.degree. C. for long periods of time, such as several tens of thousands of hours or more. One of heat-resistant metals which exhibits satisfactory strength at such high temperature is a nickel-based alloy. However, it is well known that if these devices are raised to high temperatures under even a small stress, the metal material is gradually distorted over the course of time so that it ultimately breaks down. This is referred to as the creep phenomenon. Therefore, the most important properties which the heat-resistant metal material should have are high strength and tenacity at high temperature. Further, since the devices used at such high temperatures must be constructed by welding, the creep strength of the weld joints themselves should be the same as or more than that of the heat-resistant metal itself which is a base metal. The creep strength can be represented by three important properties, i.e., the creep rate, the creep rupture time and the creep rupture ductility. For a welded structure, the weld metal itself should satisfy the requirements on the three properties. There is substantially no weld metal satisfying all of the three requirements, practically. That is, although it may be possible to obtain a weld metal where creep rupture time is substantially the same as that of the base metal, the creep rupture ductility and the creep rate of the weld metal are much smaller than those of the base metal. In view of this fact, it has been thought as generally preferable for the creep rupture time of the weld metal to be longer than that of the base metal or the creep rate thereof to be smaller than that of the base metal so that the base metal is distorted and ruptured before the distortion and rupture of the weld metal.
The weld metal forming a weld joint is first melted and then solidified. Accordingly, the micro-structure thereof is just the solidified structure which is different from that obtained by rolling of the base metal and is not metallographically stable. Therefore, even if metal material which is the same as the base metal is used as filler between the base metals welded together, the creep properties of the weld metal may be much different from those of the base metal. That is, for the welded structure, the difference in creep properties between the base metal and the weld metal causes the strength and reliability of the structure to be considerably degraded. Examples showing the above matter will be described with reference to experiments performed by the inventors.
Table 1 shows chemical components of a heat-resistant nickel-base alloy (trade name; Hastelloy X).
TABLE 1 ______________________________________ % Constituent min. max. ______________________________________ C 0.05 0.15 Cr 20.05 23.00 Co 0.50 2.50 Fe 17.00 20.00 Mn 1.00 Mo 8.00 10.00 P 0.04 Si 1.00 S 0.03 W 0.20 1.00 Ni balance ______________________________________
A creep test sample 1 in the shape of a circular rod as shown in FIG. 1 made of the heat-resistant nickel-base alloy shown in Table 1, was prepared. The diameter, and cross sectional area of the rod were 6 mm and 28.26 mm.sup.2, respectively. A welded portion 2 of the rod 1 was formed by inert gas shielded tungsten arc welding (which herein refers to TIG). The test sample 1 was stretched axially at 900.degree. C. and a time length from an application of a load stress to a rupture was measured. The result is shown in FIG. 2 in which a broken line 3 shows the result for a solid rod made of the heat-resistant nickel-base alloy shown in Table 1 having the same dimensions as these of the test sample 1 but having no welded portion and a solid line 4 shows the result for the test sample 1. FIG. 2 clearly shows that there is substantially no difference in the rupture time between the solid rod and the test sample 1. For the test sample 1, rupture occurred at the welded portion. However, when the temperature was 1000.degree. C., the rupture occurred in the base metal portion of the test sample 1 which might show that the welded portion exhibits a sufficient creep strength. However, this can not be applied to a structure having such welded portions.
FIG. 3 is a cross sectional view of a cylindrical test sample 2 which was prepared by butt-welding a pair of cup shaped base metal members 5 made of the heat-resistant nickel-base alloy shown in Table 1 by using the TIG to form a welded portion 6 therebetween. One end of the test sample 2 is closed and the other end is opened through a small pipe 7. The sample made of the heat-resistant nickel-base alloy shown in Table 1, having the same shape and dimensions as those of the test sample 2 as shown in FIG. 3 but having no welded portion was also prepared for comparison purpose. Gas pressure was applied through the pipe 7 into an interior of the comparative sample and the test sample 2 to establish inner pressures of 27.5 kg/cm.sup.2, 34.5 kg/cm.sup.2 and 45 kg/cm.sup.2, respectively. When a comparative sample was put at high temperature, it was inflated as shown in FIG. 4 and after it was inflated enough small cracks 8 occurred through which gas leeked out.
The test sample 2 was also inflated in the manner shown in FIG. 5, since the creep rate of the welded portion 6 was smaller than that of the base metal portion. Further since the creep rupture ductility of the welded portion 6 was small, cracks were produced abruptly without distortion of the welded portion. Since the creep rate of the base metal is considerably larger than that of the welded portion, the distorted base metal portion may pull the welded portion, so that rupture of the welded portion is expedited. The rupture of the cylindrical structure having the welded portion occurred within a short time, i.e., one half to one tenth the time required for the rupture of the cylindrical structure having no welded portion. In other words, the reliability of the welded structure for high temperature use depends upon a matching of the three creep properties of the base metal and weld metal.
As another experiment, a plate was prepared by TIG-welding together a pair of plate pieces a made of the heat-resistant nickel-base alloy shown in Table 1 as shown in FIG. 6 using a filler consisting essentially of the heat-resistant nickel-base alloy shown in Table 1, in which b shows a welded portion. By machining the thus obtained plate, a test sample c was subjected to a creep-test under a load stress of 4.5 kgf/mm.sup.2 at 900.degree. C. At the same time, a solid sample made of the heat-resistant nickel-base alloy shown in Table 1, having the same dimensions as those of the test sample c and having no welded portion was prepared and tested under the same conditions. FIG. 7 shows the results of this experiment. In FIG. 7, a curve 13 shows characteristics of the test sample c having the welded portion and a curve 14 shows characteristics of the sample having no welded portion. With respect to the test sample c having welded portion, since the creep rate of the welded portion b is smaller than that of the base metal a, the amount of creep strain over the test sample c is smaller than that of the sample having no welded portion. However, the creep rupture may occur in either the base meral portion a or the welded portion b. The creep rupture duration is substantially common for the test sample c and the sample having no welded portion.
Further, a cylindrical test sample 3 having the same dimension as in the test sample 2 as shown in FIG. 3 was prepared by TIG butt-welding a pair of cup shaced base metal members d made of the heat-resistant nickel-base alloy shown in Table 1 using a filler consisting essentially of the heat-resistant nickel-base alloy shown in Table 1, so that the resultant test sample 3 had a center welded portion e. A cylindrical sample made of the heat-resistant nickel-base alloy shown in Table 1 which has no welded portion was also prepared. A creep test was conducted for these two samples by applying an inner pressure load corresponding to 4.5 kgf/mm.sup.2.
FIG. 9 shows the results of this experiment. In FIG. 9, a curve 15 shows the result for the test sample 3 shown in FIG. 8 and a curve 16 shows that for the sample having no welded portion. FIG. 9 clearly shows that the test sample 3 having the welded portion e is ruptured within a short period of time and thus it is clear that both of the creep strength and creep strain thereof are very small ccmpared with those of the sample having no welded portion. This is due to the fact that the weld metal is pulled by one base metal portion where the creep rate is larger than that of the welded portion and that the welded portion exhibits a small elongation at break, i.e., the welded portion quickly reaches an allowable elongation.
The creep property of such welded structure at high temperature depends upon the weld metal. Therefore, in order to improve the strength and tenacity of the heat-resistant metal material to be used for a structure which requires weldings, it is necessary to provide an improved filler.
It has been generally thought that when boron is added to a heat-resistant nickel-base alloy or a austenite. stainless steel, M.sub.2 B.sub.2 and/or M.sub.3 B.sub.4 where melting point is low is formed around columnar crystal interface and for this reason the weld cracking occurs. Further when boron is added to structural material for nuclear reactors, the structure becomes brittle with neutron irradiation. Therefore, the amount of boron to be added thereto should be as small as possible. It has been known that the creep strength of weld metal of heat-resistant nickel-base alloy varies depending on the fusing method and conditions of the filler.